1. Field of the Invention
This invention relates to the manufacture of flat glass according to a process in which molten glass is delivered along a delivery facility and onto the surface of a pool of molten metal which supports it during forming. More particularly, this invention relates to a method for initiating such a process and establishing an initial body of glass on molten metal in a forming chamber when a forming chamber is operated for the first time or following periods of shutdown.
2. Brief Description of the Prior Art
It is well known that following the construction of a new glassmaking facility employing a forming chamber that the forming chamber and its associated equipment must be initially heated to a temperature within a suitable range for receiving molten glass for forming prior to the manufacture of flat glass by such a process in the forming chamber. It is also well known that following periods of shutdown, either for maintenance or otherwise, it is necessary to re-establish desired thermal conditions within a forming chamber and its attendant molten glass delivery facilities prior to commencing the manufacture of flat glass therein. In addition to initially establishing desired thermal conditions within a forming chamber, it is also known to be necessary to establish desirable non-oxidizing conditions within a chamber employing molten metal as a glass support. Otherwise, molten metal upon which glass is to be supported in such a process will oxidize. During steady state operation that is established following an initial startup period, it is known that maintenance of a non-oxidizing atmosphere within such a glass forming chamber is essential. The following patents disclose the introduction of non-oxidizing or reducing gases into an enclosed chamber in which flat glass is to be formed by floating molten glass on molten metal and thereafter cooling the glass sufficiently to form it into a dimensionally sheet of flat glass.
U.S. Pat. No. 1,553,773 discloses the introduction of nitrogen to provide a non-oxidizing atmosphere or environment in which flat glass is to be formed by a float method.
U.S. Pat. No. 2,911,759 discloses the use of coal gas as a non-oxidizing atmosphere for a float process. Coal gas, of course, contains methane and hydrogen as reducing agents.
U.S. Pat. No. 3,241,937 discloses use of a mixture of nitrogen and hydrogen (93 percent nitrogen and 7 percent hydrogen) as an atmosphere for a float process. This patent additionally shows the introduction of atmosphere gas into the glass forming chamber along a line transverse the entrance portion of the forming chamber as seen along section line 5--5 in FIG. 1 of this patent.
Initially, during the startup procedure to ready a flat glass forming chamber for receiving molten glass, it is common to establish a heating schedule which provides for increasing the temperature of the forming chamber from the outside ambient temperature to a temperature of from 900.degree. to 1500.degree. F. (480.degree. to 820.degree. C.) over a period of several days and to purge the interior of the forming chamber with a reducing gas, particularly toward the end of the heating schedule when the temperature is above 500.degree. to 600.degree. F. (260.degree. to 320.degree. C.). Meanwhile, it is common to add tin metal to the forming chamber so that it melts to form a pool of molten metal during the period of heating the chamber. When the chamber has reached a suitable temperature within the suggested range, the forming chamber is commonly allowed to soak at that temperature while reducing gas is fed to the chamber and the condition of the molten metal is observed.
Initially, the molten metal commonly has metal oxide floating on its surface. Metal oxide is referred to as dross. After a period of soaking at the high temperature, the metal oxide is reduced. In some instances it is found expedient to draw the floating dross or metal oxide mechanically from the surface of the molten metal and to physically remove it as a solidified dross from the chamber. During heatup it is a common problem to encounter excessive metal oxide in the vicinity of the refractory portions of molten glass delivery facility which extend into the inlet end of the forming chamber and it is not uncommon to encounter some contamination of such refractory pieces by metal oxide which adheres strongly to the surfaces. Various expedients have been attempted to isolate the inlet end of a forming chamber from the remaining portions of a forming chamber during heatup in order to minimize the contamination of such refractory pieces with metal oxide. For example, curtains and other of refractory materials and plates of refractory materials and metal have been employed to physically segregate the entrance end of a forming chamber from the remainder of the forming chamber during heatup. It has been an objective to find a suitable means for isolating the entrance or inlet end of a forming chamber from the remainder of the chamber during heatup, which means may be expediently removed after the initiation of molten glass flow into the chamber. It is also desired to provide for positively maintaining the cleanliness of the inlet end of the forming chamber prior to the introduction of molten glass into it. The present invention is believed to provide a method for accomplishing these objectives.